"North Korea has started submarine drills and stepped up preparations for nationwide military exercises," Yonhap reported, adding that North Korea's drills may start just as the US and South Korea begin their yearly joint military exercises next week.

The news follows Pyongyang's declaration on Tuesday that it would "completely declare invalid" the armistice pact that put an end to the Korean War in 1953.

This is not the first time North Korea has threatened such action, as CNN points out.

"In the aftermath of a previous nuclear test in 2009, it said its military would no longer be bound by the agreement because South Korea was joining a US-led anti-proliferation plan," CNN reported.

But GlobalPost senior correspondent Geoffrey Cain says there's more than meets the eye with these threats.

"Usually, when North Korea wants to pull off something sinister, it doesn't tell anybody that it's preparing to enforce a no-fly and no-sail zone off both its coasts," Cain said from Seoul.

In 2010, when the regime wanted to terrify the South Koreans, they simply shelled a South Korean island, killing four, and supposedly torpedoed a South Korean naval corvette, killing 46 sailors.

"So there's something else at work here," Cain continued. "It's likely that because Park Geun-hye took the helm as the South Korean president two weeks ago, the North doesn't want to launch acts of brinkmanship quite yet."

"It would be prefer to simply carry off a show of strength — a so-called military exercise — rather than actually kill South Koreans."

"Should it do the latter," Cain said, "there's little chance that Kim Jong Un will get South Korean aid in his coffers for the next five years, a tit-for-tat that could seriously damage the ability of the North Korean government to feed itself."

Following North Korea's recent successful nuclear test, China, North Korea's only major ally, the US and other nations have agreed to tighten sanctions. The Christian Science Monitor reported that strict economic sanctions are expected to be unanimously approved by the UN Security Council on Thursday.

In the last few days North and South Korea have exchanged alarming official statements, as the North boasted of its nuclear arsenal on Tuesday.

"If North Korea attempts a provocation that threatens the lives and security of our people, our military will forcefully and decisively strike not only the origin of provocation and its supporting forces but also its command leadership," said Kim Yong-hyun, chief operations officer at the military’s Office of Joint Chiefs of Staff. "We make it clear that we are all prepared."

Technically, North and South Korea have been at war for decades. Their civil war from 1950-53 ended with a truce, not a peace treaty.